1. Field of the Invention
This invention relates to a manufacturing method for an optical information recording medium and a manufacturing device for an optical information recording medium, and is for instance applicable to light exposure devices for base disks. The present invention regulates the temperature of the semiconductor laser by compensating for chromatic aberrations or compensating for fluctuations in the wavelength so that with a laser beam from a laser-pumped semiconductor laser, a base disk can be exposed at high precision.
2. Description of the Related Art
In the process of the related art for manufacturing a base disk, after exposing the disk in the exposure device, an optical disk is manufactured by a stamper. The stamper further produces the base disk in mass quantities and an optical disk is produced after forming a protective film on the base disk.
A perspective view of the optical disk is shown in FIG. 8. After an information recording surface 3 is formed on the base disk 2, a protective film 4 is formed to produce an optical disk 1.
The base disk 2 is disk-shaped member of transparent plastic. Tiny irregularities (convex and concave shapes) are formed on the information recording surface side of this base disk 2. These tiny irregularities are set as various shapes according to the manufacture of the optical disk. In an optical disk for record and reproduction such as the initial minidisk, groove shapes are formed by a laser beam to constitute the guide grooves shown by arrow A in the enlarged view in FIG. 8. In an optical disk solely for reproduction such as a compact disc, concave shapes are formed to constitute the pits shown by arrow B in the enlarged view of FIG. 8. In initial MO (magnetic optical) disks made to ISO standards, both pits and grooves are formed.
Among the optical disks 1 capable of recording/reproducing, phase-change optical disks have an information recording surface formed of laminations of phase-change layers and reflective layers on the surface of the base disk 2 formed with these tiny irregularities. On optical magnetic disks, the information recording surface is formed of laminations of magnetic layers and reflective layers. On an optical disk 1 solely for reproduction (playback), an information recording surface is formed of a reflective layer on the surface of the disk substrate 2.
A diagrammatic sketch of the manufacturing process for the optical disk for producing the disk substrate 2 is shown in FIGS. 9A to 9F. In the manufacturing process for this optical disk, the surface of a glass substrate 5 is ground (polished) flat, the glass substrate 5 washed (FIG. 9A), and a photoresist 6 applied by spin coating (FIG. 9B) to the surface of the glass substrate 5. The photoresist 6 here is applied in a thickness of approximately 100 nm, using a material that is alkali-soluble when exposed to light. The manufacturing method for the optical disk in this way produces a base disk 7 from this glass substrate 5.
Next, in the optical disk manufacturing process, the disk base 7 is set in the exposure device and the disk base 7 driven to rotate at a specific speed (FIG. 9C). While in this state, a laser beam L1 as the exposure light, is focused by means of an objective lens 6 on the photoresist 6 on the disk base 7, and along with modulating the exposure laser beam L1 by means of a modulating signal, the beam position of the exposure laser beam L1 is shifted sequentially to the outer circumference. The scanning track of the exposure laser beam L1 is in this way formed in a spiral shape in the optical disk manufacturing process, and a latent image formed according to the modulation signal in this scanning track.
The latent image formed on the base disk 7 in this way in the optical disk manufacturing process is developed (FIG. 9D) and the portions of the photoresist 6 exposed to light are dissolved away by the developer fluid. In this way, the tiny irregularities are formed on the surface of the base disk 7. The example in FIG. 9D shows that the base disk 7 is formed with the tiny irregularities corresponding to the grooves and lands.
In the next step (FIG. 9E) of the process for manufacturing the optical disk, after a nickel plating layer 8 is formed by nickel (Ni) plating on the side formed with the tiny irregularities, this nickel plating layer 8 is then peeled away from the base disk 7. In this way, in the optical disk manufacturing process, the tiny irregularities of the base disk 7 are transferred to the nickel plating layer, and a frame made by the nickel plating layer 8 then set in a metal mold to make a stamper 9.
Next, in the process for manufacturing the optical disk, the disk substrate 2 is made by the plastic injection molding or the so-called 2P method (photo polymerization) using the stamper 9 (FIG. 9F). The tiny irregularities of the base disk 7 transferred to the stamper 9 are now transferred to the disk substrate 2. The latent image of the tiny irregularities described using FIG. 8 is in this way formed on this disk substrate 2 by exposure of the base disk 7 to light in the exposure device.
A flat view showing the exposure device used in light exposure of the base disk 7 is shown in FIG. 10. A diagrammatic sketch for describing the optical system of an exposure device 11 is shown in FIG. 11. The exposure device 11 contains a base disk 7 on a turntable 12, driven to rotate in a specific direction shown by the arrow A. The exposure device 11 drives an optical drive table 13 radially across the base disk 7 as shown by the arrow B. In this way, the exposure device 11 makes an exposure laser beam (LR) scanning track in a spiral shape on the base disk 7 by means of the optical system contained in the optical drive table 13, and makes a latent image consisting of arrays of pits in the scanning track.
A laser light source 14 in the exposure device 11 is a gas laser comprising Ar, Kr, Hexe2x80x94Cd, etc. An (exposure) laser beam LR is beamed within a wavelength of 500 nm and quantity of light of 50 mW to expose the photoresist on the base disk 7 to light. When the laser light source 14 is for a Kr laser, the (exposure) laser beam LR is beamed at a wavelength within 413 nm.
An electro-optical crystal element 15 and an optical detector element 16 compensate (offset) fluctuations of the luminous energy in the (exposure) laser beam LR and emit the beam. In other words, the electro-optical crystal element 15 changes the polarized plane of the (exposure) laser beam LR emitted by the laser light source 14 according to a drive signal and the optical detector element 16 selectively permeates the specified polarized surface components. Next, a beam splitter 17 separates the (exposure) laser beam LR into two beams and outputs these beams and the optical receive element 18 receives the (exposure) laser beam LR on the side permeated by the beam splitter 17 and outputs the detected quantity of light (luminous energy).
A recording optical power control circuit 19 (FIG. 11) generates a drive signal so that the signal level with the light quantity detection results from the optical receive element 18 match a reference voltage REF and drives the electro-optical crystal element 15. The electro-optical crystal element 15 thus forms a feedback loop along with the optical detector element 16, the beam splitter 17, the optical receive element 18, and the optical receive element 18 and maintain the luminous energy (hereafter, quantity of light) of the (exposure) laser beam LR at a fixed luminous energy level.
The electro-optical crystal element 15 along with a feedback loop having a frequency response with an upper limit of 1 [MHz], reduces the noise of the (exposure) laser beam LR.
A lens 21 (FIG. 10) converts the side of the exposure laser beam LR reflected by the beam splitter 17 into a concentrated light beam and outputs it to an AOM (acousto-optic modulator) 22. The AOM22 is driven by a record signal corresponding to the latent image formed on the base disk 7, and performs on/off modulation of this (exposure) laser beam LR. Next, a lens 23 converts the light emitted from the AOM22 into parallel light rays.
Next, the polarized beam splitter 24 bends the optical path of the (exposure) laser beam LR emitted from the lens 23 and emits the beam. The xc2xc wave plate 25 applies a phase change to the (exposure) laser beam LR and emit a circular polarized light.
A beam expander 28 comprises a lens 26 and a lens 27 and expands the diameter of the (exposure) laser beam LR beamed from the xc2xc wave plate 25 and emits the beam. When the focus distance of the lens 26 and lens 27 are respectively set as f1 and f2, the beam expander 28 expands the beam diameter of the (exposure) laser beam LR by f2/f1.
An objective lens 30 receives the (exposure) laser beam LR by way of a dichroic prism 29 and a mirror not shown in the drawing, and focuses the (exposure) laser beam LR on the resist layer of the base disk 7. The optical system with components from the beam expander 28 to the objective lens 30, along with the focus control optical system described later on, are installed in the optical drive table 13 of the exposure device 11. In the exposure device 11, the movement of the optical drive table 13 shifts the light exposure position so that a latent image of pit arrays are formed on the base disk 7.
When the (exposure) laser beam LR is beamed in this way in the exposure device 11, a returning light is acquired from the base disk 7. This returning light follows the reverse of the (exposure) laser beam LR optical path and is linearly polarized by the xc2xc wave plate 25. The returning light consequently permeates a beam splitter 24. A mirror 31 bends the optical path of the returning light permeating through the beam splitter 24, and a lens 32 then guides the returning light into an image device 33. The image device 33 receives the returning light and outputs the received light results. The exposure device 11 can in this way monitor the beam shape of the exposure laser beam on the base disk 7 and is capable of adjusting control items such as focus control.
The focus control of the optical system on the other hand, detects the distance to the base disk 7 by means of the so-called isolated axis method. In other words, the focus control of the optical system is installed in the optical drive table 13 and beams out a focus control laser beam LF by means of the laser light source 35.
A polarized beam splitter 36 reflects the laser beam LF. Next, a xc2xc wave plate 37 applies a phase change to this laser beam LF and emits a circular polarized light. A dichroic prism 31 reflects the laser beam LF beamed from the xc2xc wave plate 37 to combine and emit it with the (exposure) laser beam LR constituting a permeable light.
The focus control optical system is set in this way to separate the optical axis of laser beam LF combined with laser beam LR, from the optical axis of the objective lens 30 by a specific distance. The laser beam LF is in this way, beamed diagonally onto the base disk 7 and, the optical axis of the reflected light from laser beam LF undergo a regular (specular) reflection on the base disk 7, and are separated from the objective lens 30 optical path according to the distance between the base disk 7 and the objective lens 30.
A dichroic prism 31 reflects the reflected light obtained in this way from the base disk 7 and emits the beam onto the xc2xc wave plate 37. By applying a phase differential to this reflected light, the xc2xc wave plate 37 emits a reflected light from the polarized plane intersecting with the input light of the laser beam LF.
The reflected light next permeates the polarized beam splitter 36 and the position detector element 39 receives this reflected light and outputs a position detection signal according to the change in signal level relative to the position of the received light. The exposure device 11 shifts the objective lens 30 in the direction of the optical axis so that the position detection signal reaches a specified level. Focus control is thus performed so that a latent image can be stably formed by pit arrays.
However the exposure device of the related art has the problem that the laser light source is a gas laser so that large size equipment is unavoidable. The number of exposure devices 11 that can be installed at the manufacturing site is therefore limited by the available installation space so that the optical disks cannot be produced in sufficient quantities. The laser light source of the gas laser incidentally, has approximate dimensions of a length of 1.2 m, a weight of 40 kg, and the exposure device has approximate dimensions of a width of 1.5 to 2.0 m, a depth of 1.0 to 1.2 m, a height of 1 m and a weight of 2 t.
Further problems are that the optical system has a complicated structure, which causes a large equipment size. Time is also required to adjust the optical system.
The optical path of the exposure laser beam LR becomes longer because of the complex structure of the optical system, rendering unavoidable effects from shimmer or turbulence in the air along the optical path so that the accuracy of the light exposure precision tends to deteriorate.
The gas laser further requires liquid cooling. The vibration from the flow path for the cooling fluid is conveyed to sections such as the optical drive table 13 which also adversely affects the accuracy of light exposure precision.
Use of a semiconductor laser was considered in order to resolve all these problems with the related art. More specifically, using a semiconductor laser allows the laser light source to be made more compact, and the overall shape of the light exposure device can be made compact. Further, the output beam from the laser light source can be directly modulated, so that the optical modulating element (lens 21 and 23 in FIG. 10 and AOM 22, 23) can be eliminated and the optical system given a simpler structure. The overall shape of the light exposure device can therefore be reduced and further the task of adjusting the optical system can be simplified. Simplifying the structure of the optical system makes the length of the optical path shorter by a corresponding amount so that a decline in exposure precision due to shimmer or turbulence in the air around the optical path can also be avoided. A yet further advantage is that that cooling fluid is not required so that a decline in exposure precision due to vibration conveyed from the fluid path can be prevented.
During actual use however, secondary modes are present in the semiconductor laser so that the coherence is poor compared to a gas laser and the laser beam tends to spread out plus or minus several nm versus the center wavelength.
In a laser beam having this kind of spreading, the focus position at each wavelength differs when focusing with an objective lens in a phenomenon where chromatic aberrations occurs, and consequently, a small diameter beam spot cannot be made to occur even by focusing the light up to the refraction index of the laser beam. Chromatic aberrations differ according to the wavelength for the refraction rate of the glass or plastic lens material however, generally the shorter the wavelength, the larger the chromatic aberration.
The material of the objective lens contains a low dispersion glass material such as FCD-1, FCD-10, 434-950. Even with this low dispersion glass however, at an infrared light on a wavelength of 800 nm, the focus point will deviate approximately 70 nm when using an objective lens in the light exposure device of the related art, for every one nanometer that the wavelength differs. Therefore in an exposure laser beam with a wavelength in the vicinity of 400 nm, the amount of deviation becomes even larger.
In the light exposure device 11 on the other hand, an objective lens with a high number of apertures (N.A.) is used of about 0.9. In such a lens with a high number of apertures the depth of the focus point is exceedingly shallow. The focus point depth in other words, is shown by xc2x1xcex/(2xc3x97(NA)2) and when the number of apertures (N.A.) is 0.9, an (exposure) laser beam LR oh a wavelength of approximately 400 nm has a focus depth of xc2x1250 nm.
In contrast, in an optical disk with a recording density for example approximately that of a DVD, the defocus amount allowed by defocus control during exposure is experientially known to be approximately one-third of the focus point depth. When the amount of defocus exceeds this figure, the signal waveform of the recording signal drastically deteriorates. Thus, even if the spread at the wavelength of an exposure laser beam is assumed to be approximately xc2x11 nm, then deterioration of the reproduction signal is likely to occur. Therefore, deterioration in the signal wave form of the reproduction signal is unavoidable when the semiconductor laser comprises the laser light source.
The semiconductor laser further has the disadvantage that the center wavelength fluctuates with changes in temperature. This fluctuation in the center wavelength varies according to the semiconductor material including the semiconductor laser but in for instance an AlGaAs type semiconductor laser with a center wavelength of approximately 835 nm, the center wavelength will change approximately xc2x15 nm for package temperature fluctuations of xc2x120xc2x0 C.
In the light exposure device 11, when the center wavelength of an (exposure) laser beam of this type fluctuates, the (exposure) laser beam defocuses by a corresponding amount on the base disk, the diameter of the beam spot formed on the base disk spreads out, and achieving light exposure with high precision becomes difficult.
Also, by changing the phase differential applied such as with a xc2xc wave plate, the quantity of light (luminous energy) of the permeable light, and reflected light is changed on optical elements having a light detection plane such as a polarized beam splitter, and consequently, the light quantity of the exposure laser beam focused on the base disk is changed. Incidentally, when exposing a DVD pattern to light with an (exposure) laser beam on a wavelength of 413 nm and an optical system with a number of apertures (N.A.) of 0.90, the fluctuations in light quantity of the exposure laser beam must be held within 5 (%Pxe2x80x94P), however according to the conditions of the optical system, a light quantity of this amount will fluctuate just by a change of several nm in the wavelength.
Achieving high light exposure accuracy is therefore difficult when the laser light source is a semiconductor laser. Even if a latent image is formed with the desired, specified high precision, recording at a maximum short bit length of 0.40 nm, and a track pitch of 0.74 nm as used in DVD recording is impossible.
In view of the above problems with the related art, this invention has the object of providing a manufacturing method for an optical information recording medium and a manufacturing device for an optical information recording medium capable of exposing a base disk to light at high precision with a laser light source comprised by a semiconductor laser.
To resolve the above problems with the related art, a manufacturing method for an optical information recording medium or a manufacturing device for an optical information recording medium of this invention includes a chromatic aberration compensation optical system to compensate for chromatic aberrations in the objective lens for at least the (exposure) laser beam.
To further resolve the problems of the related art, in a manufacturing method for an optical information recording medium or a manufacturing device for an optical information recording medium according to one aspect of the present invention, a laser-pumped semiconductor laser emits a laser beam for light exposure, wherein temperature variations in the vicinity of the laser light source are maintained within xc2x11xc2x0 C. by a specified temperature regulator mechanism when the wavelength of the laser beam for light exposure is less than 500 nm.
To also resolve the problems of the related art, a manufacturing method for an optical information recording medium or a manufacturing device for an optical information recording medium according to another aspect of the present invention, is provided wherein the temperature of the semiconductor laser is controlled so the wavelength of the exposure laser beam is a fixed wavelength.
To yet further resolve problems of the related art, a manufacturing method for an optical information recording medium or a manufacturing device for an optical information recording medium according to another aspect of the present invention is provided, wherein the optical system from the semiconductor laser to the objective lens is a sealed space.
To also resolve the problems of the related art, a manufacturing method for an optical information recording medium or a manufacturing device for an optical information recording medium of according to another aspect of the present invention is provided, wherein the optical system from the semiconductor laser to the beaming of the exposure laser beam onto the objective lens is maintained as one integrated piece by a holding member of an integrated exposure optical system.
To further resolve the problems of the related art, a manufacturing device for an optical information recording medium of according to another aspect of the present invention is provided with a holding member of an exposure optical system for holding the semiconductor laser, automatic light quantity regulator means and optical system in one integrated, replaceable piece.
To still further resolve the problems of the related art, a manufacturing device for an optical information recording medium according to another aspect of the present invention is provided with an imaging means to capture an image of the returning light isolated by the light isolator means and output the imaging results and, a light quantity detection means to receive the light returning from the light isolator means and output the light quantity detection results.
According to another aspect of the present invention, by installing a chromatic aberration compensation optical system to compensate for the chromatic aberration of at least the objective lens for the exposure laser beam, the occurrence of chromatic aberrations can be prevented even in exposure laser beams with a widened wavelength, a tiny beam spot formed, and a sufficient margin for defocusing can be obtained.
According to another aspect of the present invention, a laser-pumped semiconductor laser emits a laser beam for light exposure, and when the wavelength of the exposure laser beam for light exposure is less than 500 nm, temperature variations in the vicinity of the laser light source are maintained within plus or minus one degree by a specified temperature regulator mechanism so that changes in the wavelength of the exposure laser beam can be sufficiently reduced.
Also, according to another aspect of the present invention, by controlling the temperature of the semiconductor laser so that the wavelength of the exposure laser beam becomes a fixed wavelength, a decrease in light exposure precision due to fluctuations in the wavelength can be prevented.
Further, according to another aspect of the present invention, by installing the optical system from the semiconductor laser to the objective lens in a sealed space, disturbances and turbulence from the inflow of outside air can be prevented and a drop in light exposure precision can be prevented.
Still further, according to another aspect of the present invention, by maintaining the optical system from the semiconductor laser to the beaming of the exposure laser beam onto the objective lens in a holding member of one light exposure optical system, the task of adjusting the optical system can be simplified and the assembly task simplified by a corresponding amount so that maintenance and servicing is significantly improved.
Yet further, according to another aspect of the present invention, by comprising a holding member of an exposure optical system for holding the semiconductor laser, the automatic light quantity regulator means and the optical system in one integrated, replaceable piece, the task of adjusting these mechanisms can be simplified, the assembly task simplified by a corresponding amount, and the maintenance and servicing significantly improved.
Even further, according to another aspect of the present invention, by comprising an imaging means to capture an image of the returning light isolated by the light isolator means and output the imaging results and, a light quantity detection means to receive the light returning from the light isolator means and output the light quantity detection results, convenience is provided since exposure is performed while monitoring the current status, and the task of adjusting the optical system can be further simplified.